JP2008175173A - Air-fuel ratio control device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an exhaust emission control device substantially improving catalyst efficiency, by leading, to a catalyst, exhaust gas mixed with reducing agent entirely and uniformly. <P>SOLUTION: The exhaust emission control device comprises: an exhaust emission control catalyst disposed to an exhaust passage of an engine; a detection means disposed to an exhaust passage and detecting ammonia in the exhaust gas; and an air-fuel ratio control means suppressing a degree of richness of an air-fuel ratio when the detection means detects a predetermined amount or more of ammonia. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

The present invention relates to an air-fuel ratio control device for an internal combustion engine, and more particularly to a technique for reducing the amount of ammonia (NH ₃ ) contained in exhaust gas.

  Exhaust gas discharged from the engine of vehicles such as automobiles contains many pollutants that may adversely affect the environment, such as carbon monoxide (CO), hydrocarbons (HC), and nitrogen oxides (NOx). ing. For this reason, in general, an exhaust purification catalyst such as a three-way catalyst or NOx storage catalyst is disposed in the exhaust passage through which exhaust gas discharged from the engine passes, and the exhaust gas is released into the atmosphere after being purified. To be.

  By the way, in a gasoline engine, as operation modes, stoichiometric operation in which the air-fuel ratio is in the vicinity of the stoichiometric air-fuel ratio (stoichio), rich operation in which the air-fuel ratio is richer than stoichio, and the air-fuel ratio is leaner than stoichio. By combining with a certain lean operation, the exhaust gas is efficiently purified (for example, see Patent Document 1).

  Note that during stoichiometric operation, in general, there is a case where the engine is slightly shifted to a richer side (rich shift) than stoichiometric in order to suppress the NOx emission amount. That is, even during stoichiometric operation, there may be a case where the vehicle is substantially rich.

JP 2001-132514 A

However, there is a problem that ammonia (NH ₃ ) is generated by a chemical reaction in various catalysts such as the above three-way catalyst and exhausted into the atmosphere as exhaust gas during rich operation including stoichiometric operation that is richly shifted in this way. is there. In particular, when an engine with a large amount of HC discharged, such as an in-cylinder gasoline engine, is employed, the amount of ammonia produced by the catalyst also increases.

  The amount of ammonia discharged is not currently subject to automobile exhaust gas regulations, but if the concentration is high, problems such as the generation of malodor will occur. Less is desirable.

The present invention has been made in view of such circumstances, and an object of the present invention is to provide an air-fuel ratio control apparatus capable of significantly suppressing ammonia (NH ₃ ) emission while suppressing NOx emission. To do.

  A first aspect of the present invention that solves the above problems includes an exhaust purification catalyst provided in an exhaust passage of an engine, a detection means provided in the exhaust passage for detecting ammonia in the exhaust gas, and the detection means. And an air-fuel ratio control means for performing control so as to suppress the richness of the air-fuel ratio when ammonia exceeding a fixed amount is detected.

  In the first aspect, the amount of ammonia discharged can be suppressed to an extremely low level while preventing NOx discharge.

  According to a second aspect of the present invention, there is provided the air-fuel ratio control apparatus according to the first aspect, wherein the detection means is a NOx sensor provided on the downstream side of the exhaust purification catalyst.

  In the second aspect, it is not necessary to install an ammonia detection sensor or the like, and it does not lead to a situation in which the structure is complicated and the cost is increased.

  The third aspect of the present invention further includes operation mode determination means for determining an operation mode of the engine, and the air-fuel ratio control means determines that the air-fuel ratio of the engine is a stoichiometric air-fuel ratio (stoichiometric) by the operation mode determination means. ) Or the rich non-lean operation state, and the control is performed when the output of the NOx sensor is equal to or higher than a predetermined value.

  In a fourth aspect of the present invention, when the air-fuel ratio of the engine is determined to be in a lean operation state by the operation mode determination means and the output of the NOx sensor is equal to or greater than a predetermined value, the lean operation state of the engine is prohibited. Alternatively, the air-fuel ratio control apparatus according to the third aspect further includes limiting means for limiting.

  In the fourth aspect, the amount of ammonia discharged can be suppressed to an extremely low level, and NOx discharge can be more reliably prevented.

The fifth aspect of the present invention further includes a rear O ₂ sensor provided on the downstream side of the exhaust purification catalyst, and the air-fuel ratio control means is configured so that a detection value of the rear O ₂ sensor falls within a predetermined range. The air-fuel ratio control apparatus according to any one of the first to fourth aspects is characterized in that the above control is performed.

  In the fifth aspect, it is possible to accurately detect NOx and ammonia in the exhaust gas, and to more reliably suppress emissions of these NOx and ammonia.

In the present invention, it is possible to suppress the generation of ammonia (NH ₃ ) in the catalyst and to suppress the discharge amount of ammonia extremely while suppressing the discharge amount of NOx satisfactorily. Further, by adjusting the air-fuel ratio using the output value of the NOx sensor as a trigger, it is possible to determine the amount of ammonia discharged relatively accurately, and to suppress the amount of ammonia discharged more reliably.

  Hereinafter, an embodiment of the present invention will be described with reference to the drawings.

  FIG. 1 is a diagram showing a schematic configuration of an engine system including an air-fuel ratio control apparatus according to the present invention. Hereinafter, in this embodiment, the case where this air-fuel ratio control device is applied to a direct injection type spark addition type gasoline engine will be described as an example.

  A gasoline engine (hereinafter simply referred to as an engine) 11 shown in FIG. 1 is an in-cylinder injection type spark-added gasoline engine, and includes a cylinder head 12 and a cylinder block 13. A piston 15 is accommodated in each cylinder 14 of the cylinder block 13 so as to be reciprocally movable. The piston 15, the cylinder 14, and the cylinder head 12 form a combustion chamber 16. The piston 15 is connected to the crankshaft 18 via a connecting rod 17. The reciprocating motion of the piston 15 is transmitted to the crankshaft 18 via the connecting rod 17.

  An intake port 19 is formed in the cylinder head 12. An intake manifold 20 is connected to the intake port 19, and an intake pipe 21 is connected to the intake manifold 20. An intake valve 22 is provided in the intake port 19, and the intake port 19 is opened and closed by the intake valve 22. Further, an exhaust port 23 is formed in the cylinder head 12. One end of an exhaust manifold 24 is connected to the exhaust port 23, and an exhaust pipe 25 is connected to the other end of the exhaust manifold 24. The exhaust port 23 is provided with an exhaust valve 26, and the exhaust port 23 is opened and closed by the exhaust valve 26 in the same manner as the intake valve 22 in the intake port 19.

  A spark plug 27 is attached to the cylinder head 12 for each cylinder. Each spark plug 27 is connected to an ignition coil 28 that outputs a high voltage. Further, the cylinder head 12 is provided with an electromagnetic fuel injection valve 29 together with a spark plug 27. Each fuel injection valve 29 is connected to a fuel supply device (not shown) having a fuel tank via a fuel valve. In each combustion chamber 16, the fuel in the fuel tank is directly injected from each fuel injection valve 29.

  The intake pipe 21 on the upstream side of the intake manifold 20 is provided with a throttle valve 31 for adjusting the amount of intake air, and a throttle position sensor (TPS) 32 for detecting the valve opening degree of the throttle valve 31 is also provided. ing. Further, an air flow sensor 33 for measuring the intake air amount is interposed upstream of the throttle valve 31.

In the exhaust pipe 25 connected to the exhaust manifold 24, a three-way catalyst 34 and an NOx storage catalyst 35 constituting an exhaust purification catalyst are interposed. The three-way catalyst 34 has platinum (Pt), rhodium (Rh), or palladium (Pd) as an active noble metal on a carrier. The active noble metal has an oxygen adsorption function (O ₂ storage function) in the case where it contains an oxygen storage material such as cerium (Ce) or zirconia (Zr) and also in the case where such an oxygen storage material is not included. Yes. Therefore the three-way catalyst 34, the exhaust air-fuel ratio (exhaust A / F) adsorbs oxygen (O ₂₎ in an oxidizing atmosphere is lean, the O ₂ to the exhaust A / F becomes reducing atmosphere becomes richer Hold as storage O ₂ . Then, the storage O ₂ is released (supplied) when the reducing atmosphere is reached, so that the dissociation O is removed, and HC (hydrocarbon) and CO (carbon monoxide) are oxidized and removed even in the reducing atmosphere state. It has become.

The NOx occlusion catalyst 35 disposed downstream of the three-way catalyst 34 has a noble metal such as platinum (Pt) or palladium (Pd) supported on a honeycomb structure carrier made of, for example, aluminum oxide (AL ₂ O ₃ ). In addition, an alkali metal such as barium (Ba) or potassium (K) or an alkaline earth metal is supported as a storage agent. The NOx occlusion catalyst 35 temporarily occludes NOx in an oxidizing atmosphere. For example, NOx is released in a reducing atmosphere containing carbon monoxide (CO), hydrocarbon (HC), etc., and nitrogen (N ₂ ) or the like. To reduce.

Downstream of the NOx storage catalyst 35, a rear O ₂ sensor 36 for detecting the oxygen concentration in the exhaust gas, and zirconia type NOx sensor 37 for detecting the amount of NOx in the exhaust gas is disposed. Furthermore, in the present embodiment, a front O ₂ sensor 38 that detects the O ₂ concentration in the exhaust gas is disposed upstream of the three-way catalyst 34. Although the details will be described later, in the present embodiment, by adjusting the air-fuel ratio based on the output value of the NOx sensor 37, and extremely low emissions of ammonia (NH ₃₎ suppresses the emission of NOx.

The ECU (electronic control unit) 39 includes an input / output device, a storage device (ROM, RAM, etc.), a central processing unit (CPU), a timer counter, and the like. The ECU 39 performs comprehensive control of the air-fuel ratio control device 10 including the engine 11. On the input side of the ECU 39, various sensors such as the above-described TPS 32, air flow sensor 33, rear O ₂ sensor 36, NOx sensor 37, front O ₂ sensor 38, and crank angle sensor 40 that detects the crank angle of the engine 11. Are connected, and detection information from these sensors is input.

  On the other hand, the output side of the ECU 39 is connected to various output devices such as the fuel injection valve 29, the ignition coil 28, and the throttle valve 31 described above. To these various output devices, the fuel injection fee, the fuel injection time, the ignition timing, etc. calculated by the ECU 39 based on the detection information from various sensors are output. Specifically, the air-fuel ratio is set to an appropriate target air-fuel ratio (target A / F) based on detection information from various sensors, and an amount of fuel corresponding to the target A / F is injected at the proper timing. The throttle valve 31 is adjusted to an appropriate opening degree, and spark ignition is performed at an appropriate timing by the spark plug 27.

  In such an engine 11, for example, a rich operation in which the air-fuel ratio is richer than the stoichiometric air-fuel ratio (stoichio), a stoichio, and a lean operation in which the air-fuel ratio is leaner than the stoichiometric air-fuel ratio are appropriately switched according to operating conditions and the like. As a result, NOx emissions are reduced while improving fuel efficiency. For example, in the present embodiment, a lean operation and a stoichiometric operation (non-lean operation) in which the air-fuel ratio is shifted to a slightly rich side (rich shift) in the vicinity of the stoichiometry are switched according to the operation conditions and the like.

Further, during the stoichiometric operation, the air-fuel ratio is feedback-controlled (stoichiometric feedback control) based on information from the front O ₂ sensor 38. That is, control is performed to periodically oscillate the air-fuel ratio with respect to a predetermined reference value, so-called center A / F. During the stoichiometric operation, the center A / F is normally shifted slightly richer than the stoichio (rich shift) in order to suppress the NOx emission amount, and the air-fuel ratio is not lean during the stoichiometric operation. The non-lean operation state is maintained.

  Hereinafter, the generation mechanism of ammonia in exhaust gas and the detection / suppression method will be described with reference to FIGS.

For example, when a stoichiometric operation with a relatively high degree of richness is continuously performed during engine warm-up operation, a large amount of ammonia (NH ₃ ) is discharged. FIG. 2 shows the elapsed time (sec) after engine start, the air-fuel ratio (A / F: first stage), the total hydrocarbon amount (THC (% C): second stage), and the ammonia emission amount (NH ₃ An example of the relationship with (ppm): the third stage) is shown, but from the third stage graph of FIG. 2, a peak indicating the generation of ammonia in the exhaust gas is almost detected at the inlet of the three-way catalyst 34. It can be seen that a peak appears at the outlet of the NOx storage catalyst 35. From this, it is considered that ammonia is produced by the three-way catalyst 34 and the NOx storage catalyst 35, and in particular, it is considered that a large amount of ammonia is produced by the three-way catalyst 34 for the following reason.

That is, when the air-fuel ratio (exhaust air-fuel ratio) of the exhaust gas flowing into the three-way catalyst 34 is leaner than the stoichiometric air-fuel ratio, NOx in the exhaust gas passes through the three-way catalyst 34 as it is, but flows into the three-way catalyst 34. If the air-fuel ratio of the exhaust gas is richer than the stoichiometric air-fuel ratio, NOx in the exhaust gas is converted to ammonia (NH ₃ ) in the three-way catalyst 34 based on the following reaction formulas (1) and (2). Conceivable.

NO + HC + H ₂ O → CO ₂ + NH ₃ (1)
2NO + 5H ₂ → 2H ₂ O + 2NH ₃ (2)
Further, hydrogen _{(H 2)} in the formula (2) is thought to be reduced by the following reaction formula (3) (4).

CO + H ₂ O → CO ₂ + H ₂ (3)
HC + H ₂ O → CO ₂ + CO + H ₂ (4)
During rich shift stoichiometric operation, the amount of HC flowing into the three-way catalyst 34 becomes relatively large, so that NOx flowing into the three-way catalyst 34 is easily converted to NH ₃ , and the amount of ammonia contained in the exhaust gas increases. Conceivable. In particular, as in the present embodiment, when the engine 11 is an in-cylinder injection type, the HC amount (TCH) contained in the exhaust gas discharged from the engine 11 is, for example, 0. 0 at the three-way catalyst 34 inlet. It is considered that the amount of ammonia produced by the three-way catalyst 34 and the like is further increased because it is relatively high at about 6% C (see the second graph in FIG. 2).

As described above, ammonia (NH ₃ ) contained in the exhaust gas is not currently subject to exhaust gas regulations, but if the concentration is high, problems such as generation of malodor may occur. For this reason, in the present invention, the discharge amount of ammonia is suppressed without increasing the NOx amount in the exhaust gas.

  In the present embodiment, the state of ammonia generation in the exhaust gas is determined by a so-called zirconia type NOx sensor 37 provided on the downstream side of the NOx storage catalyst 35, and the air-fuel ratio is adjusted based on the determination result, that is, By adjusting the air-fuel ratio by a control routine (air-fuel ratio control means) described below, the ammonia discharge amount (production amount) is suppressed. As described above, since the state of ammonia generation is determined by the NOx sensor 37 that is originally used for detecting NOx, it is not necessary to install an additional sensor or the like for detecting ammonia. It does not lead to a situation that causes high.

Note that the change in the output value of the NOx sensor 37 shows a certain tendency although it slightly changes depending on the operating conditions. For example, FIG. 3 shows an example of the relationship between the output value (V) of the rear O ₂ sensor and the NH ₃ emission amount (ppm) based on the output value of the NOx sensor. As shown in FIG. 3, the output value of the NOx sensor 37 is gradually decreased with increase in the output value of the rear O ₂ sensor 36, increases rapidly when the output value of the rear O ₂ sensor 36 exceeds a predetermined value It shows the change of being. The increase in the output value of the NOx sensor 37 after the output value of the rear O ₂ sensor 36 exceeds the predetermined value is not due to the increase in NOx in the exhaust gas but due to the detection of NH _3. I can say that. It can be judged from the graph shown in FIG. 4 that ammonia is detected by the NOx sensor.

FIG. 4 shows an elapsed time (sec) after engine start, an air-fuel ratio (A / F: first stage), an ammonia emission amount measured by a predetermined measuring device (NH ₃ (ppm): second stage), a predetermined It is a graph which shows an example of the relationship with the NOx discharge | emission amount (NOx (ppm): 3rd step | paragraph) obtained by the analyzer of. In the third graph, both the output value of the NOx sensor and the NOx concentration in the exhaust gas obtained by a predetermined analyzer are shown. According to FIG. 4, in the time zone (about 30 to 240 seconds) in which the A / F is richly shifted in the first graph, the detected value of the NOx sensor shown in the third graph and 2 The concentration of ammonia drawn in the graph in the second stage is almost the same, and the NOx concentration obtained by the analyzer in the third stage is almost constant and zero.

Normally, NOx is discharged in a large amount when the air-fuel ratio is lean, and since it is purified by the catalyst in a state where the air-fuel ratio is richly shifted, it should not be discharged so much. Nevertheless, the output value of the NOx sensor is obtained in the time zone during which the rich shift is performed (about 30 to 240 seconds). This means that the NOx sensor is not reacting to NOx but is NH _3. Can be thought of as reacting to As described above, this is supported by the fact that the output value of the NOx sensor and the ammonia concentration substantially coincide with each other and the actual NOx concentration is almost zero.

  Hereinafter, the air-fuel ratio control method according to the present embodiment will be described based on a flowchart showing the control routine of FIG.

  As shown in FIG. 5, first, for example, various operating conditions such as vehicle speed, engine rotation speed, volumetric efficiency, intake air flow rate, exhaust temperature, elapsed time after startup, and coolant temperature are detected (step S1).

Next, it is determined whether or not each sensor such as the front O ₂ sensor 38, the rear O ₂ sensor 36, and the NOx sensor 37 has been activated (step S2). Whether or not each sensor is activated can be determined based on, for example, an elapsed time after starting. If it is determined that each sensor has been activated (step S2: YES), then the temperatures of the three-way catalyst 34 and the NOx storage catalyst 35 are detected (step S3). The catalyst temperature may be such that the temperature of the three-way catalyst 34 and the NOx storage catalyst 35 is directly measured by a temperature sensor or the like, for example, but may be a temperature estimated based on various conditions such as the elapsed time after starting. Good.

Thereafter, the process waits until the catalyst temperature becomes equal to or higher than the predetermined temperature (step S4: NO). When the catalyst temperature becomes equal to or higher than the predetermined temperature (step S4: YES), the output value of the NOx sensor 37 serving as the detection means is detected. (Step S5). Next, in step S6, it is determined based on information from the rear O ₂ sensor 36 whether or not the current operation mode is a stoichiometric or rich non-lean operation state (operation mode determination means). In the present embodiment, it is determined whether or not the operation mode is stoichiometric operation and the center A / F is richly shifted.

If it is determined that the vehicle is in a non-lean operation state (rich shift stoichiometric operation) (step S6: YES), then the output value of the NOx sensor 37 detected in step S7 is a predetermined value C1, for example, 50 ppm. It is judged whether it is larger than (step S7). As described above, NOx should not be exhausted much when the air-fuel ratio is in a non-lean operation state. Nevertheless, the fact that the output value of the NOx sensor 37 has been obtained is considered that the NOx sensor 37 is not reacting with NOx but is reacting with NH ₃ . For this reason, when the output value of the NOx sensor 37 is larger than the predetermined value C1 (step S7: YES), it is determined that the output value of the NOx sensor 37 is due to ammonia.

If it is determined in step S7 that the NH ₃ control start condition is satisfied (step S7: YES), NH ₃ suppression control is performed (step S8). Specifically, for example, the air-fuel ratio is adjusted by increasing or decreasing the fuel injection amount or the intake air amount from the fuel injection valve 29, and the richness of the air-fuel ratio is suppressed to the extent that lean operation is not performed. At this time, the adjustment of the air-fuel ratio may be performed based on the output value of the front O ₂ sensor 38, but is preferably performed based on the output value of the rear O ₂ sensor 36. In the present embodiment, the air-fuel ratio is adjusted so that the output value of the rear O ₂ sensor 36 is within a predetermined range. Of course, the air-fuel ratio may be adjusted based on the output values of both sensors.

For example, the air-fuel ratio is adjusted so that the output value R-O _{2 of} the rear O ₂ sensor 36 satisfies 0.70 V <R-O ₂ <0.84 V. Specifically, for example, when the output value of the rear O ₂ sensor 36 is 0.80 ≦ R−O ₂ based on the NH ₃ and NOx emission characteristics shown in FIG. If it is determined that the output value is due to ammonia, and the output value of the rear O ₂ sensor 36 is 0.84 ≦ R−O ₂ , the air-fuel ratio is slightly leaned, and the output value of the rear O ₂ sensor 36 is 0.70V _<R-O 2 _<suppress rich degree of the central a / F such that the 0.84 V. As a result, the amount of ammonia discharged can be suppressed to an extremely low level while suppressing the amount of NOx discharged.

Here, as shown in FIG. 6, the NH ₃ emission amount (detected by the NOx sensor 37) is obtained by suppressing the lean degree of the air-fuel ratio during stoichiometric operation, that is, the rich degree of the center A / F. It has been found that by suppressing the gradual reduction. On the other hand, as shown in FIG. 7, the NOx emission amount increases slightly as the air-fuel ratio richness is suppressed, but does not change greatly. Therefore, by suppressing the richness of the air-fuel ratio when ammonia is detected by the NOx sensor 37, it is possible to reliably suppress the ammonia emission amount while reducing the NOx emission amount to a small amount.

Further, in the present embodiment, the air-fuel ratio is adjusted based on the output value of the rear O ₂ sensor 36 provided in the vicinity of the outlet of the most downstream NOx storage catalyst 35 constituting the exhaust purification catalyst. Therefore, NOx in the exhaust gas can be reliably removed, and the amount of NOx in the exhaust gas is extremely small. Therefore, the NOx sensor 37 substantially detects only ammonia. In other words, by adjusting the air-fuel ratio based on the output value of the rear O ₂ sensor, the ammonia emission characteristic is greatly affected by the deterioration state of the catalyst, but the ammonia concentration can be accurately monitored regardless of the state of the catalyst. Can do. Therefore, it is possible to reliably suppress ammonia emission while preventing NOx emission.

Of course, the adjustment of the air-fuel ratio in the NH ₃ suppression control does not have to be performed once, and may be performed a plurality of times stepwise.

Further, when the NH ₃ suppression control is performed in step S8, if the output value of the rear O ₂ sensor is R−O ₂ <0.70 V, the center A / F is further richly shifted. It may be. Thereby, the amount of NOx emission can be further suppressed.

Thereafter, in step S9, it is determined whether or not the ammonia suppression control end condition is satisfied. For example, when the output value of the NOx sensor is detected again and the output value is lower than the predetermined value C1, it is determined that the ammonia suppression control condition is satisfied (step S9: YES), and the NH ₃ suppression control is stopped ( Step S10) and return to step S3.

  If it is determined in step S6 that the current operation mode is lean operation (step S6: NO), it is then determined whether the output value of the NOx sensor 37 is equal to or greater than a predetermined value C2 (step S11). As described above, NOx is detected by the NOx sensor 37 during the lean operation, not ammonia. Therefore, when the output value of the NOx sensor 37 is greater than or equal to the predetermined value C2, that is, when NOx greater than or equal to the predetermined amount is detected (step S11: YES), the lean operation is prohibited or restricted in step S12. (Restriction means). Thus, after NOx emission is prevented, the process returns to step S1. On the other hand, when the output value of the NOx sensor 37 is smaller than the predetermined value C2 in step S11 (step S11: NO), the process directly returns to step S1.

  In this embodiment, as described above, ammonia is detected by the NOx sensor 37 based on the knowledge that the NOx sensor 37 detects ammonia instead of NOx under a certain condition. However, the original use of the NOx sensor 37 is to detect NOx in the exhaust gas during the lean operation as in step S11.

Although one embodiment of the present invention has been described above, the present invention is not limited to this embodiment. For example, in the above-described embodiment, the rear O ₂ sensor is provided in the vicinity of the most downstream catalyst outlet constituting the exhaust gas purification catalyst. However, the present invention is not limited to this. For example, the amount of ammonia generated is relatively large. You may make it provide in the exit vicinity of a three-way catalyst. Further, for example, in the above-described embodiment, the present invention has been described by exemplifying an in-cylinder injection type engine. Of course, the present invention is not limited to other types such as an intake pipe injection type (MPI). It can also be used for engines. Furthermore, the present invention can be applied to a system equipped with a NOx storage catalyst in a diesel engine.

It is a schematic block diagram of the air fuel ratio control apparatus which concerns on one Embodiment. It is a graph showing the relationship between the elapsed time after startup and NH ₃ concentration and the like. Is a graph showing an example of the relationship between the rear O ₂ sensor output values and the output value of the NOx sensor. It is a graph which shows the relationship between elapsed time after starting, ammonia discharge amount, NOx discharge amount, etc. It is a flowchart which shows an example of the control routine of the air fuel ratio control method which concerns on one Embodiment. It is a graph which shows the relationship between the richness of an air fuel ratio, and the amount of ammonia. It is a graph which shows the relationship between the richness of an air fuel ratio, and the amount of NOx.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Air-fuel ratio control apparatus 11 Engine 12 Cylinder head 13 Cylinder block 14 Cylinder bore 15 Piston 16 Combustion chamber 17 Connecting rod 18 Crankshaft 19 Intake port 20 Intake manifold 21 Intake pipe 22 Intake valve 23 Exhaust port 24 Exhaust manifold 25 Exhaust pipe 26 Exhaust valve 27 Spark plug 28 Ignition coil 29 Fuel injection valve 30 Fuel valve 31 Throttle valve 33 Air flow sensor 34 Three-way catalyst 35 Storage catalyst 36 Rear O ₂ sensor 37 NOx sensor 38 Front O ₂ sensor 39 ECU
40 Crank angle sensor

Claims (5)

An exhaust purification catalyst provided in the exhaust passage of the engine;
A detecting means provided in the exhaust passage for detecting ammonia in the exhaust gas;
An air-fuel ratio control means for performing control so as to suppress the rich degree of the air-fuel ratio when the detection means detects ammonia of a predetermined amount or more;
An air-fuel ratio control apparatus comprising:   The air-fuel ratio control apparatus according to claim 1, wherein the detection means is a NOx sensor provided downstream of the exhaust purification catalyst. An operation mode determination means for determining an operation mode of the engine;
The air-fuel ratio control means is determined by the operation mode determination means that the engine air-fuel ratio is in a stoichiometric or rich non-lean operation state, and the output of the NOx sensor is greater than or equal to a predetermined value. The air-fuel ratio control apparatus according to claim 2, wherein the control is performed.   When the operation mode determination means determines that the air-fuel ratio of the engine is in a lean operation state and the output of the NOx sensor is equal to or greater than a predetermined value, it further has a restriction means for prohibiting or limiting the lean operation state of the engine. The air-fuel ratio control apparatus according to claim 3. A rear O ₂ sensor provided on the downstream side of the exhaust purification catalyst;
5. The air-fuel ratio control apparatus according to claim 1, wherein the air-fuel ratio control unit performs the control so that a detection value of the rear O ₂ sensor falls within a predetermined range.

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